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Two diamond disks with different surface roughness were used. Ten cylindrical ceramics were reduced with each diamond disks. The time required for the same amount of reduction of ceramic block was measured as a milling efficacy. The wear of the diamond disk was also measured.The results were as follows :1. For the coarse diamond disk, the milling efficacy did not drop after ten uses. On the other hand, the fine diamond disk showed rapid drop to approximate 40% comparing with a new diamond disk.2. The wear loss after milling 10 blocks were 24.2 μm at the surface level and 22.5 μm at the edge level for the coarse diamond disk. The fine diamond disks showed wear of 3.0 μm at the surface and 1.2 μm at the edge.3. By SEM observation of the coarse diamond disks, dull edge and loss of diamond particles were more remarkable than those of the fine disks.